The following account of the prior art relates to one of the areas of application of the present application, hearing aids (including headsets).
The battery power in a hearing aid lasts as little as 3 days for a conventional Zinc-air battery and as little as 6 hours for a rechargeable solution (depending on the battery size and the power consumption of the hearing aid). In order to make the battery power last as long as possible, the user should turn off the hearing aid when it is not in use, i.e. when it is not placed in or at the ear. This is today done by either opening the battery drawer, or by operating a switch on the hearing aid.
To open the battery drawer for powering off can be a problematic issue for users with reduced dexterity. In hospitals and nursing homes, care personnel often have to switch off hearing instruments that have been taken off but not powered down by their owner.
An additional switch, used for powering off, takes up space in hearing aids that in many cases are designed to be as small as possible.
There are two reasons why the hearing instruments should be powered off:
1) When hearing instruments are powered on while not used, the battery lifetime is reduced unnecessarily.
2) When hearing instruments are powered on while not used, they may annoy other people in the environment with a feedback sound that often occurs when a hearing instrument is turned on, but not worn.
The automatic provision of a power off mode in a hearing aid or other hearing assistance device (with the aim of automatically powering the hearing instrument off, when it is not worn) is thus attractive and has been dealt with in a number of prior art documents, some of which are identified in the following.
U.S. Pat. No. 4,955,729 discloses the use of a sensor for deciding whether or not a hearing aid should be switched on or off. The hearing aid includes an electronic amplifier, an electric power source and a switch for automatically breaking or making the connection between the amplifier and the power source depending on whether the hearing aid is in use or out of use. The switch is provided in such a manner so as to be responsive to a switching criterion defined by a change of state such as change in temperature, moisture etc. Sensors for measuring change of temperature, moisture, light, posture, oxygen partial pressure, motion, feedback (the latter identifying a signal generated through acoustic feedback between microphone and earphone after removing the hearing aid) are mentioned.
US 2005/0226446 A1 deals with a hearing aid that is capable of automatically switching between a full-function mode and a sleep mode depending on the location of the hearing aid. The hearing aid comprises a location sensor module for providing a location information signal to indicate one of an in-the-ear case and an out-of-the-ear case. Location information is based on using the surface reflection of IR light (e.g. 600-800 nm) by human skin to switch between a full function mode and a sleep mode of a hearing aid.
U.S. Pat. No. 7,522,739 deals with the switching on and off of a hearing aid using a temperature sensor, a pressure sensor, or a resistance sensor to detect an electrical load resistance as a function of volume or an acoustic sensor to detect a sound level.
U.S. Pat. No. 6,532,294 discloses the use of a temperature sensor or a contact sensor for detecting whether or not a hearing aid should be switched on or off.
EP 0 674 466 A1 discloses the use of an acoustic sensor, a temperature sensor, a photo detector, a force sensor, or a resistance sensor for detecting whether or not a hearing aid should be switched on or off.
EP 1 465 454 A2 describes detecting removal of a hearing aid from the ear canal by measuring the receiver signal “reflected” from the ear canal.
US 2009/087005 A1 describes a pair of wirelessly connected hearing aids that are automatically switched on and off based on a field strength or value of an electromagnetic signal received by a hearing aid that is transmitted from the respective other hearing aid.
DE 10 2008 054087 A1 describes a hearing aid comprising a capacitive proximity sensor comprising two metallic electrodes. The proximity sensor is designed, such that the hearing aid is switched off when the hearing aid is not worn at a head. The electrodes are formed by structuring an inner side of the housing.
EP 2 071 873 A1 decides whether a hearing aid is worn or not, by actively sending out a measurement signal and comparing the measured properties of the acoustic path transfer function (feedback) with reference data (stored in a memory) collected while the hearing was being worn under normal conditions. Whenever this comparison shows significant (predefined) differences, it is automatically concluded that the hearing aid is currently not being worn and an automatic power-off to conserve the battery is triggered.
U.S. Pat. No. 5,144,678 describes a headset with an on/off switch, which can turn itself on or off depending on whether or not it is placed on the head of a user.
U.S. Pat. No. 7,010,332 describes a wireless headset with automatic on/off-function. Various (general) sensors are mentioned, incl. proximity-sensors.
US 2006/0029234 A1 describes a ‘headphone device’ comprising a system for detecting whether or not it is in use, with the aim of identifying the ‘state’ of the device. A temperature-sensor and a skin-resistance-sensor are specifically mentioned.
US 2006/0233413 A1 describes the use of a capacitance sensor in an ‘ear-phone’-system for on/off-control.
US 2008/0080705 A1 describes a headset comprising means for detecting whether or not it is mounted on the head of a user. The use of an IR-sensor for this purpose is mentioned.
U.S. Pat. No. 6,704,428 describes detecting removal of a headset when noise generated by blood-flow or jaw-movement in the user's head disappears.
DE 4034096 discloses detecting non-use of a mobile device (e.g. a hearing aid) by means of a motion sensor. The mobile device (or at least one stage of such device) is switched ON and/or OFF in dependence on movement or a movement change as detected by a movement responding sensor.
EP 2 211 579 A1 discloses a portable communication system comprising first and second communication devices, the system being adapted to detect when the two communication devices are located closer to each other or farther from each other than a range indicating a normal distance of operation and corresponding to a VeryClose and a VeryFar zone, respectively, and to use the dynamic transmit power regulation to implement a partial power-down mode of the system, when the two communication devices are located in said VeryClose or in said VeryFar zone.
EP 2 071 873 A1 mentions the possibility of putting a hearing instrument in a low power mode, based on monitoring the acoustic feedback path by means of a waveform and a matched filter that is adapted to it.
EP 1 871 140 B1 describes the use of a hearing aid comprising a coil and a current measurement unit configured to measure a current in the coil and an external resonance circuit (e.g. located in a storage box) allowing the measurement unit to measure a change in current when the hearing aid is located in the vicinity of the resonance circuit, and to perform an action based thereon, e.g. to power the hearing aid off. The use of a magnetic switch in a hearing aid and an external magnet in a storage box for the same use (power down) is also mentioned.
GB 1254017 A describes a reed relay switch inside a hearing aid which is operable by a magnet outside the casing to switch the hearing aid on and off. At night the hearing aid is placed in a case and a permanent magnet opens the reed relay switch to disconnect the accumulator from the amplifier.
US 2007/253584 A1 describes a binaural hearing aid system comprising first and second hearing devices. The first and second hearing device each comprise a permanent magnet and a magnetic field sensor such that they can be switched off, when the first and second hearing devices are located in close physical proximity to each other.
Some of these schemes have the disadvantage that the measurement used to decide whether the device is worn or not has the potential to disturb the user of the device or other people.
Most of the prior art solutions rely on a single measurement (or on the value of a single parameter) and may at times lead to erroneous conclusions due to unforeseen situations. A forced power-down at an unintended point in time may of course be highly frustrating for a user.